1. Field of the Invention
This invention is to provide a kind of photothermosetting composition which is composed of a base resin, a photopolymerizable photomonomer or photooligomer, a photoinitiator, an epoxide compound, and an organic solvent. The composition may be coated on the substrate and cured to form a solder mask.
2. Description of the Prior Art
Printed circuit board (PCB) is frequently applied to various electronic products. After finishing circuits of PCB, it needs to be coated with a solder mask (ink, so-called green paint) or solder resist where it does not need to solder to avoid adhesion of solder during the assembly of an electronic device on the circuit board, to seal for moisture resistance and to be an insulator between an electronic device and a conductive wire, to avoid damage to the circuit board from dust, finger print, and moisture air.
Basically, the solder masks are divided into four different types as follows:
(1) Ultraviolet curable: using screen printing for a single-sided board primarily;
(2) Thermosetting: using screen printing for a double-sided board primarily;
(3) Liquid photoimageable solder mask (LPISM): using screen printing, is the most popular method of solder mask at present; and
(4) Dry film photoimageable solder mask.
The coating method for the solder mask can be divided into screen printing, curtain coating, spray coating, roller coating, dip coating, and lamination; and among them the methods of screen printing and curtain coating are much more popular.
Due to the development of high density interconnect (HDI) and high level packaging technique the requirements for a liquid photo solder mask are also very severe, such as non-tacky after prebake; no pressed scar after cooling and exposing to form a mask; a mask must be rigid after postbake; have a good adhesion property; much higher insulation resist; wide process window of prebaking temperature as well as development conditions, the smaller and the better undercut; high resolution and easily rework for stripping; at least 260xc2x0 C. for the heat resistance; higher glass transition temperature (Tg); 2-3 mil ({fraction (1/1000)} inch) resolution for solder dam; chemical resistance and acid and alkaline resistance.
It needs a chemical treatment after postbaking of solder mask, such as a hot air solder leveling (HASL), an electroless platting, a chemical Tin-immersion, or an organic solderability preservatives (OSP) and golden finger are less used. European Union (E.U.) has legislated to require environmental protection; hence, in March of 1999 E.U. has already passed the second edition of draft in title of xe2x80x9cWaste Electrical and Electrical Equipmentxe2x80x9d (WEEE) which prohibits use of lead metal in electronic products from January of 2004. Therefore, solder mask materials must increase its chemical resistance and heat resistance to suit for the new process, i.e. in the leadless process of chemical Tin-immersion, replacement of HASL, uses very strong acid (pH less than 1); thus, if solder mask does not have good acid resistance and adhesion property, it is susceptible to bleeding or peeling problems of the solder mask to influence the qualification. Up to date the known solder masks do not meet the required acid resistance and adhesion property during the leadless chemical Tin-immersion process.
In general, LPISM contains xe2x80x9cmain agentxe2x80x9d, so-called main part, and xe2x80x9chardener agentxe2x80x9d, so-called curing part, which is able to produce a cross-link effect with main part. The components of these two agents, however they gather together to be used directly (one liquid type) or they separate before use followed by mixing together, are composed of:
(1) Possessing carboxylic acid group and photopolymerizable double bond of a base resin (A);
(2) Photopolymerizable double bond of monomer or oligomer, (or called photomonomer and photooligomer) (B);
(3) Photoinitiator, (or called photoactive compound) (C);
(4) Curing agent.
As U.S. Pat. Nos. 4,722,947; 4,75,138; 5,087,552; 5,114,830; 5,296,334; and 6,007,966 demonstrated that the modified styrene-maleic anhyhride (SMA) could be used as the base resin; in the process of this kind of resin the side product of water could be produced and remained in the base resin during the esterification. The residual water in the base resin will not facilitate the property of solder mask, and also will decompose the acid anhydride to increase the acid value ( greater than 200 mg KOH/g), and will cause uncontrolled problems. Hence, if the acid value is controlled in the range of available value, it has to have a high ratio of unreacted acid anhydride to remain in the base resin, but it will not facilitate the property of solder mask; thus, U.S. Pat. Nos. 4,722,947 and 5,087,552 mentioned that it needs to collect the water or to add dehydrating agent to decrease the amount of the residual water in resin during the preparation of SMA resin. However, good results could not be obtained for the electroless-platting or chemical Tin-immersion process if SMA-containing solder mask was used.
Besides, U.S. Pat. Nos. 4,943,516; 5,009,982; 5,100,767; 5,538,821; and 5,753,722 demonstrated that the acrylic acid-modified epoxy resin, could be used as the base resin, which composed of carboxylic group as well as unsaturated double bond in the side chain of the polymer backbone; or as U.S. Pat. No. 5,821,031 presented that the acrylic acid-modified acrylate copolymer, which composed of carboxylic group as well as unsaturated double bond in the side chain of the polymer backbone. Since the ring opening reaction of the epoxide compound for this kind of photo component was very slow and was not completed, it needed to use the excess of toxic unsaturated monobasic acid (e.g. acrylic acid and its derivatives) to accelerate the reaction be completed; but, it could not avoid unreacted unsaturated monobasic acid to be remained in the component, it not only caused the damage of the human health, but also it lowered down the efficiency of platting process, even it caused the problems of bleeding and peeling during the chemical Tin-immersion process.
Furthermore, as U.S. Pat. No. 5,821,031 describes that if the curing agent contained epoxy moiety, which is sparingly soluble in the diluent, i.e. xe2x80x9csolvent-insoluble typexe2x80x9d of epoxide compound; its pellet powder needed to be ground by the grinder during the preparation of solder mask, but decrease of the particle size was limited to increase the capital of manufacture, and it had no definite efficiency for heat resistance and chemical resistance.
Hence, the primary aim of this invention is to solve the drawbacks described above. In order to avoid the presence of the drawbacks described above, this invention is to provide a kind of photothermosetting composition whose property is able to meet the requirements of a leadless chemical Tin-immersion process.
The other aim of this invention is to provide a kind of photothermosetting composition using less of an amount of unsaturated monobasic acid (e.g. acrylic acid and its derivatives), which would cause the damage of human health.
The other aim of this invention is to provide a kind of photothermosetting composition in which the epoxide compound could be firstly dissolved in the formula solvent of solder mask for spare; it omits the process of mechanical grinding resulting the powder dust, and the epoxide compound could be placed in the components of curing agent to omit the electrical consumption of grinding, to decrease capital expenditure for protecting pollution and manufacture.
The other aim of this invention is to provide a kind of photothermosetting component in which the base resin does not possess an irritant acid and also does not cause the corrosive damage for the human health due to the lower amount of the residual acid.
In order to get the results described above, this invention provides a kind of photothermosetting composition which is composed of a base resin (A) containing at least two epoxy groups of epoxide compound (a), containing at least two double bonds and an unsaturated monobasic acid (b), a saturated or unsaturated dibasic acid (c), and a saturated or unsaturated acid anhydride (d) according to the typical ratio, a photopolymerizable photomonomer or photooligomer (B), a photoinitiator (C), containing at least two epoxy groups of epoxide compound (D), and an organic solvent; which is then cured to form a solder mask.